The field of the present invention relates to data reading systems, and particularly to an optical system for reading bar codes such as found on consumer products (i.e. a UPC code), the system having improved depth of field and focusing through incorporation of micro electro-mechanical systems. The system is suitable for a variety of stationary or handheld scanners.
Bar code scanners, as any optical system, depend upon focused optics for effective and accurate performance. In a detection system such as a bar code scanning device employing a focusing lens, a light source such as a laser, laser diode, or non-coherent light source (e.g., light emitting diode) emits light which passes through and is focused by the focusing lens. The object containing the bar code is passed through the focused beam and if the bar code is sufficiently close to the beam focal point, reflected light from the bar code may be detected resulting in a successful scan. Specifically, the detected light reflected from the bar code is read by the scanner that then creates a signal based on the characteristics of the detected light. Because different barcodes create reflected light having different and unique characteristics, which are detectable by the scanner system, it is possible to assign data to a specific barcode based on the signal produced by the reflected light from the barcode.
As known by one skilled in the art, a focal point is typically not a discrete point but may be referred to as a xe2x80x9cwaistxe2x80x9d which is the position along the beam axis where the xe2x80x9cconexe2x80x9d of light from the light source reaches a minimum spot size, usually as measured in a direction parallel to the direction of spot motion.
A problem arises when the bar code or object being scanned does not fall sufficiently close to the focal point or waist, that is, when the beam spot is too large or too small to successfully read a symbol. By way of example, in a supermarket checkout application, a product bearing a UPC bar code label is passed at a certain distance in front of the window of a checkout scanner. The checkout scanner is designed with a scanning beam with a waist of a given diameter positioned at a certain distance from the window where the bar code is expected to pass. The checkout clerk must become familiar with the proper distance to pass the object in front of the window, that is, the bar code must pass sufficiently close to the scanner focal point or waist (i.e. within its depth of field) in order to achieve a successful scan.
However, in some applications, it may be desirable for the scanning device to function over a range of distances. U.S. Pat. No. 5,945,670 to Rudeen et al. discloses a variable aperture device that is electronically controllable for selectively adjusting the waist location of the outgoing beam. U.S. Pat. No. 5,438,187 to Rudeen et al. discloses using a laser beam to different distances via a focusing lens having multiple zones. U.S. Pat. Nos. 5,770,847 and 5,814,803 to Olmstead disclose image readers systems with multi-focus lenses. In systems as disclosed in U.S. Pat. No. 4,818,886, the position of the detector or the light source itself is movedxe2x80x94changing the object distance.
Another attempt at providing multiple depths of field is described in U.S. Pat. No. 4,560,862 which uses a rotatable optical polygon mirror having a plurality of facets, each mirror facet being of a different curvature. As the polygon mirror rotates, a different mirror facet reflects the beam from the light source along an optical path, each mirror facet providing a corresponding focal plane. The device multiplexes the signal to read the signal received from the various focal planes. Since the rotating polygon mirror also scans the outgoing beam, the device may also not be readily compatible with existing scanner designs and only allows a certain number of discrete focal points (one focal point for each mirror facet). Moreover, changing between selected sets of focal points would require replacing mirror facets or making some other hardware adjustment or modification.
Accordingly, the present inventor has recognized the desirability for a system for actively focusing a data reader/scanner which can change the focus parameters at low power and nearly instantaneously as the scanner reads the bar code.
The present invention is directed to a focusing system and method of focusing for a data reader, in a preferred configuration comprising a micro-optical system.
In a preferred application, the focusing system can vary the optimum waist focus distance as the bar code symbol is read in order to maintain an optimum focus and reduce or eliminate xe2x80x9cfalsexe2x80x9d reads or non-reading of the bar code symbol because the waist was either too small or too large. Furthermore, the focusing system may include a micro-optical system which utilizes advanced technology in order to make the scanner extremely compact so that the device is easily fabricated and suitable for use with such devices as pen scanners, hand scanners, wrist-mounted scanning devices, and other applications where it is desirable to have an extremely compact, robust scanning device. In one embodiment, the system comprises a scanning device that is mounted on a silicon substrate using micro-electromechanical systems (MEMS) technology.
Another embodiment comprises a scanner having an adjustable focus which could be varied xe2x80x9cin the fieldxe2x80x9d for optimum performance in a variety of applications. In this way, the scanner is varied by altering the focus parameters input by the user, wherein the focus parameters are dependent on the application for which the scanner is to be used. For example, the device may be configured to have a very small xe2x80x9cwaistxe2x80x9d or xe2x80x9cspotxe2x80x9d to read extremely small barcodes for applications where-space for barcode labels is limited or where barcodes are deliberately made unobtrusive. Alternatively, for applications where the barcode label is of poor quality, the scanner may be configured to have a large waist or spot size in order to resolve voids or ambiguities in the barcode.
In an alternative embodiment, a focusing system comprises a focus aperture that may be nearly instantaneously varied through the use of an electronic actuator. In this manner, the focus aperture is widened or narrowed to optimize for a variety of bar sizes. Such a configuration is particularly suitable for applications where there are voids in printed bars in that a large waist size better integrates over these voids and increases the accuracy of the scanner.